Originally cable networks were established to transmit television signals to homes and offices. Cable networks provide advantages over transmission television networks that include a clearer signal and a greater selection of channels. These cable networks originally were intended simply to provide customers with television signals. Therefore, they did not provide customers a means to transmit signals to the cable network operator, to other customers, or to any other party.
In early embodiments, cable networks transmitted analog signals. More recently, cable networks have been converted to transmit digital signals. With digital cable networks, customers may transmit signals back to the cable network operator. This facility has been called the upstream channel in contrast to the forward or downstream channel that delivers programming to the customer. Additionally, digital cable networks may now be coupled to the Internet thereby providing houses and offices access to the Internet. This access is generally faster than access provided by other technologies.
FIG. 1 shows a typical digital cable network. A cable modem termination system (“CMTS”) 100 is the base component of the system. The CMTS 100 is a typical system well known in the art and typically includes facilities for forward error correction. The CMTS 100 is coupled to the Internet backbone 102. The Internet backbone 102 carries Internet traffic, is well known in the art, and typically comprises fiber optic communication lines. Alternatively, the Internet backbone 102 may comprise co-axial cables. CMTS 100 is also coupled to a combiner/splitter 116. Combiner/splitter 116 combines signals from CMTS 100 and from fiber optic lines 104. Fiber optic lines 104 carry television signals. Fiber optic lines 104 may alternatively be co-axial cables. Combiner/splitter 116 is well known in the art. Combiner/splitter 116 combines signals from the Internet backbone 102 and the incoming television signals from fiber optic lines 104 and outputs a resulting signal on a fiber optic transmission line 106. Combiner/splitter 116 also receives signals from fiber optic transmission line 106 and splits off the appropriate signals for transmission to the Internet backbone 102 or for use by the cable network operator. Fiber optic transmission line 106 is a standard cable network transmission line comprising fiber optics and related equipment.
Fiber optic transmission line 106 is coupled to one or more nodes 108. In an alternative embodiment, a co-axial cable couples combiner/splitter 116 to nodes 108. Nodes 108 serve as distribution points for the cable network receiving signals from the CMTS 100 transmitted on the fiber optic transmission line 106. Each node 108 is coupled to a local area cable loop (“LACL”) 110. LACL 110 is typically a co-axial cable that carries signals from the node to homes and businesses within in a relatively close geographical area. Alternatively, LACL 110 may be a fiber optic cable. LACL 110 forms a loop with its beginning and end coupled to node 108. Node 108 retransmits signals received from combiner/splitter 116 on its associated LACL 110. In alternate embodiments, LACL 110 may be coupled directly to combiner/splitter 116. Those of ordinary skill in the art will recognize that a network may have many different structures.
Coupled to each LACL 110 are one or more customer sites 112. A customer site may be either a home or office and is coupled to LACL 110 by a co-axial cable 114 that taps off of LACL 110.
FIG. 2 shows the coupling of LACL 110 to a plurality of customer sites 112 and to node 108. Node 108 has two couplings to LACL 110, 200 and 202 respectively. Couplings 200 and 202 are logical couplings; in actuality, a single fiber optic transmission line or co-axial cable couples node 108 to LACL 110. FIG. 2 also shows a customer site in more detail. Corresponding to couplings 200 and 202 are couplings 204 and 206. Couplings 204 and 206 are logical representations of the coupling between customer site 112 and LACL 110. In actuality, the coupling between customer site 112 and LACL 110 is a coaxial cable, twisted pair, wireless, or other coupling. Coupling 204 provides a forward or downstream channel signal path from LACL 110 to customer site 112. Correspondingly, coupling 206 provides an upstream signal path from customer site 112 to LACL 110 and then to node 108 and CMTS 100.
Customer site 112 comprises a combiner/splitter 214, a cable modem 208 and at least one computer 210 and at least one television 212. Computer 210 is well known in the art and is equipped with a browser for transmitting to and receiving information from the Internet. Computer 210 is coupled to cable modem 208. Cable modem 208 is a conventional cable modem. Cable modem 208 is coupled to combiner/splitter 214 which is a conventional device for combining and splitting digital signals. Combiner/splitter 214 is similar to combiner/splitter 116 but on a scale appropriate for a customer site. Those of ordinary skill in the art will recognize combiner/splitter 214. Television 212 is also coupled to combiner/splitter 214.
Coupling 200 provides a forward channel signal path from node 108, through LACL 110, through coupling 204, and to the customer site 112. In the forward channel, the node transmits digital signals at frequencies between 88 and 860 MHz. Typically, the forward channel provides forward error correction whereby cable modem 208 is capable of detecting errors in the signal it receives from node 108. Having detected errors in the signal, cable modem 208 then corrects the errors. The methods and means for correcting signals received by a cable modem are well known in the art.
Coupling 202 provides a signal path for customer sites 112 to communicate with the cable network operator. Cable modem 208 transmits through coupling 206, through LACL 110, and through coupling 202 in the reverse direction of the forward channel through coupling 202. This upstream channel is generally limited to frequencies between 5 and 50 MHz. Historically, the cable network operators used the upstream channel to provide a signal path from customer sites 112 to the cable network operator. For example, a customer site may request specific programming, sometimes known as pay per view. A customer site may also use the upstream channel to send other requests to the cable network operator such as billing inquiries, notices of service deficiencies, and other information.
FIG. 3A illustrates the form of a signal 300 in the forward channel from combiner/splitter 116 to a customer site. The forward channel signal is the result of time multiplexing more than one serial signal. FIG. 3A shows three exemplary signals 302 that are time multiplexed into a down stream signal 300. The forward channel signal is framed with each frame lead by a header 304. The header 304 indicates the beginning of the frame and the remaining bits of the frame are the payload. The signal to each customer site is time multiplexed with the signals intended for every other customer site on the LACL and loaded into the payload of the frame. The cable modem at each customer site monitors the forward channel signal and picks off the bits in the payload of each frame that are intended for the specific cable modem. The cable modem then reconstructs the signal intended for the customer site. There are many methods of time multiplexing signals; those of ordinary skill in the art will recognize these methods.
The upstream channel is typically not framed. FIG. 3B illustrates the form of an upstream channel signal 306 that comprises packets transmitted in a token ring type protocol. When a cable modem first couples to a LACL, it receives a schedule from the CMTS. This schedule designates when the cable modem may transmit in the upstream channel. When the schedule indicates that the cable modem may transmit in the upstream channel, the cable modem composes a packet and transmits it in the reverse direction of the forward stream to the CMTS.
A packet comprises a packed identifier (“PID”) 308 and a payload 3 10. The PID 308 specifies the cable modem that transmitted the packet, specifies the destination of the packet, and may include other information. The payload 310 of the packet comprises entirely data from the transmitting cable modem. Unlike the forward channel signal, upstream channel packets are typically not multiplexed with signals from other cable modems; the entire packet payload comprises data from a single cable modem.
As mentioned above, the upstream channel was originally intended to be used for sending requests to the cable network operator and has generally not provided high transmission performance. A customer site, however, would typically transmit infrequently and then would transmit only small amounts of data. The upstream channel, therefore, was generally not crowded and performance was not a priority.
More recently, cable network operators have coupled CMTSs to the Internet backbone to provide customer sites with Internet access. Generally, such systems have offered advantages over other technologies for coupling customer sites to the Internet. The forward channel has a large bandwidth that allows for fast downloads of data to customer sites. Additionally, cable systems are already in place; removing the necessity of large capital investment. At first, the limited performance of the upstream channel was not an impediment to using cable systems for Internet access. Generally, Internet traffic from a customer site to the Internet backbone required much less bandwidth than the forward stream required. Customer sites would make requests to web sites for data. The requests, being small, would not require broad bandwidth. The requested data, however, would frequently be large and require the broad bandwidth of the forward channel. The poor performance of the upstream channel was not relevant to the use of cable systems for Internet access. If a signal from a customer site never reached its destination, the customer site simply sent the signal again. The size of signals and their number did not noticeably impede Internet access.
More recently, however, the limited performance of the upstream channel is impeding Internet access. Customer sites are transmitting more data in the upstream channel. Some customer sites now are themselves web sites or are uploading large amounts of information to the Internet. Thus, the amount of data transmitted in the upstream channel has increased. Additionally, there are more customer sites accessing the Internet; correspondingly there is more traffic in the upstream channel. The greater number of customer sites means fewer transmission slots for any one customer site; should a signal need to be retransmitted, there will be a delay until another transmission slot is available. Also, the increased size of the signals makes them more burdensome to resend. No longer is it acceptable simply to resend a signal that fails to reach its destination. In order to utilize fully the capabilities of a cable network, impairments that result in retransmission of packets in the upstream channel must be diagnosed and repaired.
It has become imperative to improve the performance of the upstream channel in cable networks. With the need to improve upstream channel performance, there is a corresponding need to test the upstream channel. Testers are needed to determine when and where the upstream channel is limiting network performance. The upstream channel is difficult to test. At this time, when a customer site is experiencing inadequate Internet access, a test set must be used at the customer site and simultaneously, a test set must be used at the CMTS, the node, or other location. The need to coordinate multiple test sets makes testing the upstream channel difficult. In addition, disabling the network while testing is not practical. There is a need for devices and methods to test accurately and efficiently the upstream channel of cable networks without disabling the network while testing.